A. Field Of The Invention
The present invention relates generally to electric water heaters, and more particularly to portable electric water heaters for use in livestock water tanks exposed to low temperature extremes. The invention further relates to an improved means for regulating the cycling of an electric heating device which substantially prevents the surface of a body of water from freezing in a water tank or the like, thereby providing simultaneous access to the water within the tank by a plurality of livestock at low temperature extremes.
B. Prior Art
Livestock require large amounts of water throughout the year. In order to provide livestock with needed water during the winter months, exterior livestock watering tanks are seasonally equipped with electric water heaters to prevent the water contained therein from icing over during temperatures below freezing (32.degree. F.).
Electric water heaters for livestock watering tanks fall into two categories; submerged and floating. In both categories of water heaters, some method for temperature monitoring and regulation is provided. Temperature monitoring and regulation is necessary to address three problems: (1) the need to cut off power to the heating element when the device is dislocated from the water trough (safety); (2) the need to prevent unnecessary heating in mild temperatures; and (3) the need to provide maximum access to the water at low temperature extremes.
Brodie (U.S. Pat. No. 2,430,272) discloses a portable floating livestock water heater which does not address the safety problem. In Brodie, temperature monitoring and regulation is achieved by means of a bimetallic thermostatic bar for operating a thermostatic switch which controls the power to the heating element. Brodie's metallic bar is located in a position of "maximum response to changes in outside ambient air temperature and to the sun's radiant heat" and has essentially no response to the temperature of the heating element (See Col. 3, ln. 13-16). Thus, the Brodie device, when displaced from the trough as by an animal, would not cut off power to the heating elements until the ambient air was sufficiently hot--possibly as a result of a straw or grass fire.
Temple (U.S. Pat. No. 2,472,178) discloses a portable floating device which responds to the temperature of the water. However, the Temple device provides only minimum access to the water at low temperatures. In Temple, the thermostatic switch monitors the water-temperature immediately below the device and is set to open at "a temperature only . . . slightly above the freezing temperature of water." (Col. 2, ln. 10-14). According to Temple, the apparatus acts to maintain the surface of the water against freezing "over an area encompassed by and extending slightly beyond the periphery of the heating element but will permit the water to freeze over the remainder of the surface". (Col. 2, ln. 15-20). Temple further discloses that in cold weather, the unfrozen area around the device is so small that the animal, which wishes to drink, is required to press its nose against the device, submerging it, so as to gain access to the unfrozen area. Thus, the Temple device, which only permits one animal at a time to drink, is unsuited for use with large herds of livestock.
Langenbahn (U.S. Pat. No. 2,511,721) discloses a self filling stock tank having a non-portable heating device which is an integral part of the tank itself. In Langenbahn, the temperature of the water is monitored indirectly by a thermostat located in an air enclosed chamber immediately above the water wherein the temperature of the air in the chamber is dependent upon the temperature of the water which is immediately below it.
Landgraf (U.S. Pat. No. 2,576,688) discloses a portable tank mounted livestock water heater which addresses both the safety problem and the problem of unnecessary heating in mild temperatures, but the problem of providing maximum access to the water at low temperature extremes.
In Landgraf, the first two problems are solved by monitoring the temperature of the heating coil, as buffered by the water temperature, rather than the air or water alone. Specifically, the heating coil temperature is monitored by means of a submerged air capsule positioned between two legs of the heating coil, wherein two submerged metal plates provided contact between both legs of the heating coil and said capsule. Operationally, as the heating coil heats up, heat from the coil is transferred through the conductive metal plates and water to the capsule, causing the air in the capsule to expand driving a bellows and a plunger. Movement of the plunger past a thermostatically pre-set point automatically cuts off power to the heating coils.
Thus, in Landgraf, the power to the heating coil is cut off when the water is no longer able to dissipate the heat transferred from the heating coils to the air capsule, as when the water temperature locally attains the set temperature, or when the device is displaced from the water and substantially undissipated heat is transferred directly from the heating coil to the air capsule by the conductive metal plates.
Despite its improvements over the prior art, the Landgraf device was only suited to maintaining "an unfrozen opening in the vicinity of the heater large enough for drinking purposes." Col. 1, ln. 11-13. Consequently, the Landgraf device failed to provide a solution to the third problem discussed above: simultaneous access to the water within the tank by a plurality of animals by preventing a substantial area of the tank's surface from freezing.
McKinstry (U.S. Pat. No. 4,068,116) discloses a portable electric water heater having an improvement over the Landgraf device. The McKinstry device eliminates the air capsule, the bellows, and the moving plunger, and runs a "temperature sensing band" (conductive metal strip) from the submerged heating coil directly to a thermal fuse and thermostat. In McKinstry, the heat from the submerged heating coil, which is conducted towards the thermostat by the single conductive metal strip, is dissipated somewhat by the adjacent water. However, like Landgraf, McKinstry does not address the need to provide simultaneous access to the water by a plurality of animals.
Ward (U.S. Pat. No. 4,599,973) was primarily concerned with improving the safety of portable water heaters disclosed by McKinstry and Landgraf. In particular, Ward (U.S. Pat. No. 4,599,973) recognized the problem in McKinstry of poor connections between the heating coil and the conductive metal strip, and further recognized that the problem would only worsen with oxidation or corrosion of the metal surfaces. Ward also preferred to reduce the watt density of the heater as to prevent accidental fires and burns to the livestock (col. 2, ln. 10-16), and to have a device that was more directly responsive to the temperature of the heating element (Col. 1, ln. 61-66), and less responsive to the temperature of the water as he suggests the McKinstry (Col. 1, ln. 55-57) and Landgraf (Col. 1, ln. 67-68 and Col. 2, ln. 1-6) devices were.
Ward's solution was to provide the bottom of his floating device with a heavy duty molded platen-like aluminum housing, having a steel jacketed heating element molded within said housing along its circumference, and having a thermostat bonded to said housing on its inside (upper) surface. In this way, Ward reduced the watt density of the heating element by utilizing the greater surface area of the aluminum housing to dissipate the heat to the water.
However, because Ward was primarily concerned with the problem of safety, Ward did not address the problem of providing maximum access to the water by a plurality of animals during periods of extreme cold.
Moreover, none of the prior art designs disclose or even suggest a design for a portable electric water heater, which is safe even when mishandled, which prevents unnecessary heating in mild temperatures, and which provides maximal access to water at low temperature extremes.